Over the past decade, there has been considerable interest and progress in the use of heavy ions such as protons and carbon in an attempt to improve the effectiveness of radiotherapy. Both have a physical advantage over photons because of their Bragg peaks, but only heavier ions such as carbon have a potential biological advantage. This has led to the claim that the most important heavy charged particle radiotherapy of the future is more likely to be with heavy ions rather than protons, and this is the premise debated in this month's Point/Counterpoint.